1. Technical Field
The present invention relates to a surface acoustic wave filter used for an electronic device and to a SAW duplexer using the filter.
2. Background Art
Recently, the variety of communication systems used for portable phones has been increased. In a US-made PCS (Personal Communication Services) system, namely one of the communication systems, the cross band between the transmission band and reception band is set to be extremely narrow, namely 20 MHz. Accordingly, a band pass filter that has a broad pass band width and a large attenuation in extremely close proximity to the pass band is greatly required. A SAW duplexer used for such an application requires small insertion loss and sufficient suppression in the band of the other party. The band of the other party of a transmitting filter of a SAW duplexer is the reception band, and the band of the other party of a receiving filter is the transmission band. Therefore, a SAW duplexer is required to have a filter characteristic having a steep frequency characteristic in the cross band.
A surface acoustic wave filter (hereinafter referred to as “SAW filter”) is well known as one of the filters having a steep filter characteristic. A SAW filter, however, has a temperature coefficient of frequency depending on an employed piezoelectric substrate. The temperature coefficient of frequency of a GHz-band SAW filter employing a generally used lithium tantalate substrate, for example, is −40 through −35 ppm/° C. Therefore, for realizing a SAW duplexer employing a SAW filter that is used in a communication system such as PCS having a narrow cross band, the temperature coefficient of frequency of the SAW filter must be improved.
The following structure is known in order to provide a surface acoustic wave device that has a relatively good temperature coefficient of frequency and broadband characteristic. In this structure, a silicon dioxide (SiO2) film is formed on the surfaces of surface acoustic wave resonators forming at least one SAW filter that is disposed on a piezoelectric substrate, and the surface acoustic wave resonators are interconnected by at least one of a series connecting method and a parallel connecting method. Thus, a target surface acoustic wave device is realized.
Japanese Patent Unexamined Publication No. 2003-60476 discloses the following surface acoustic wave device. This surface acoustic wave device has the following elements:                at least one surface acoustic wave filter formed on a piezoelectric substrate; and        one-port surface acoustic wave resonators that are formed on the piezoelectric substrate and are connected to the SAW filter by at least one of a series connecting method and a parallel connecting method.A film having a positive temperature coefficient of frequency is formed so as to cover at least one of the surface acoustic wave resonators except for the part having the SAW filter. Forming the film having the positive temperature coefficient of frequency aims to improve the temperature coefficient of frequency, suppress the degradation of insertion loss into a pass band, and widen the pass band.        
In a conventional surface acoustic wave device, however, a longitudinally coupled mode SAW filter is employed as the SAW filter formed on the piezoelectric substrate. The longitudinally coupled mode SAW filter, which has been recently significantly improved, suffers a large insertion loss compared to a ladder type SAW filter that uses a surface acoustic wave resonator as the impedance element. It is thus difficult to apply a longitudinally coupled mode SAW filter to a SAW duplexer where insertion loss is required to be small. A surface acoustic wave resonator having a silicon dioxide (SiO2) film is connected in this document, so that the insertion loss further increases.
The longitudinally coupled mode SAW filter structured in one stage hardly has sufficient suppression, so the filter is generally structured in two or more stages. In a structure of two or more stages, the insertion loss nearly doubles, and hence the application to the SAW duplexer further becomes difficult. Further, in the longitudinally coupled mode SAW filter increase of the degree of suppression is difficult on the high frequency side of the pass band. Therefore, it is difficult to employ a longitudinally coupled mode SAW filter as the transmission filter of the PCS system.
The present invention addresses the conventional problems, and provides a SAW filter that has a high temperature coefficient of frequency and suffers extremely small insertion loss into the pass band. By employing the SAW filter, a SAW duplexer is provided which has a steep frequency characteristic in the cross band and a large degree of suppression in the band of the other party and prevents a signal from leaking to the band of the other party.